Water Stress Effects on Grain Quality in the Cultivation of Rice (Oryza sativa L.)

Introduction

Rice (Oryza sativa L.) is the most important crop for human consumption. It is the main food for more than half of the world's population (Ruiz et al., 2012). In Cuba, this cereal is a basic food for the Cuban population, followed by beans (MINAG, 2014, 2015).

By tradition and food habits, Cuba is among the nations with high rice consumption, with 60 kg per capita per year. National rice production does not meet domestic demand, so more than 40% of this product, that is destined to the population’s consumption, is imported (MINAG, 2014, 2015).

Cuban economy has to import annually about 4 x 10⁵ t of white grain, which amounts to 60% of total consumption. Nevertheless, the aim is to fully satisfy the demand for this food, including tourism consumption, through the structure of the production chain and the network of scientific research, which cover an area of 16 x 10⁴ ha belonging to the state, more than 32 x 10³ ha belonging to private producers, in small plots (Polón et al., 2014).

In the cooperative production sector, with the use of regrowth, agricultural yields are reported fluctuating between 2.5 and 4.7 t. ha^-1 with excellent industrial grain quality, both crystalline grains and white belly in grains (Castro et al., 2014).

Studies on bean (Phaseolus vulgaris L.) cultivation by other researchers (Polón et al., 2012), also report an increase in grain yield and better grain quality, when water stress is applied to the crop.

One of the most important agricultural inputs for any crop and especially rice is undoubtedly water (PNUD, 2016). The reduction in the use of this input is one of the benefits of the regrowth system. In Cuba, research carried out under both, research and production conditions, shows a considerable reduction of irrigation water in this cropping system up to 40% with a middle cycle variety (Polón et al., 2014).

According to FAO, world production in 2011 reached 722 million tons compared to 700 million tons in 2010. Crops have improved in almost all rice regions of the world, thanks to an extension of cultivated areas, which would reach 164 million hectares (Barrios et al., 2016). This increase is concentrated in the main producing countries, especially China, India and Indonesia, which account for almost two-thirds of world production (FAOSTAT, 2015).

An economical way to increase the industrial yield of white rice is by increasing whole grains per ton of white rice obtained from paddy rice harvested.

The industrial quality, denominated in the national and international norm as a component of the industrial yield, is the percentage of whole grains obtained after the elaboration process in the industry (Camargo et al., 2014). In the particular case of Cuba, the specifications of the Cuban standard of quality establishes that rice consume takes a percentage of broken rice, and the lower it is, the greater the quality of the product is.

On the other hand, studying the mechanism through which moisture is retained in the mature rice grain, which strongly influences its quality, is another important aspect to evaluate. Along with this, it is not only necessary to know the evolution of water within the grain, but also the storage kinetics of the dry matter in the grain, particularly, by measuring the growth of endosperm mass during ripening (Tosquy et al., 2014).

The greater nitrogen presence in the fertilization increases the crystal appearance of the grain (little white belly), reduces the fissuring and the parting of the grain in the bleaching mill. It also influences the mechanical resistance of rice starch paste when heated and then cooled (Camargo et al., 2014). The cracking of the rice grain has been studied rigorously in many institutions of the world.

The objective of the present work was to know the effect of water stress on the industrial quality of the grain in the vegetative phenological phase of the crop in a variety of short cycle.

Methods

The experiment was conducted for four years, from 2014 to 2017 at UCTB Los Palacios, on a Hydromorphic Gley Nodular Ferruginous Petroferric soil (Hernández et al., 1999).

Treatments:

Seed density was 120 kg∙ha^-1 (MINAG, 2014).

For the development of the experiment, the commercial variety of short cycle Perla de Cuba was used.

Evaluations:

For the industrial yield of the grain, a sample of 1 kg of seed was taken, and then, the percentage of whole grains, white belly and cracked grains were determined. For these last components of the industrial yield, subsamples of 100 grains were taken and observing them with a glass magnifying the amount of grains with white belly and cracks in the grain were counted.

A randomized block design was used with six treatments, five with water stress and one control with normal irrigation according to Technical Instruction (MINAG, 2014).The water stress was applied in the vegetative phase from leaf wilting to yellowing, and the soil totally cracked.

The data obtained were submitted to a simple variance analysis, applying Duncan's multiple range scores when significant differences were found among the means for the significance level (p≤0.05).

Determination of total nitrogen. Rationale: The traditional (and most accurate) method for determining N is based on converting all forms of it to ammoniac nitrogen which in acid medium has the NH₄ + form and is stable. Then an alkaline medium is propitiated to pass it to NH₃, distill it and collect it in acid medium and value it, but it is a very long method and requires a lot of dedication. Therefore, for the determination in a simpler way, the colorimetric method is utilized with the reagent of Nessler (Paneque, 2010).

Results and Discussion

There are many factors that affect rice yield and its industrial quality, among them, the time of harvesting and the irrigation management prior to harvest, resulting in decreasing of the percentage of whole and cracked grains and grains with white belly (Thompson y Mutters, 2006). However, in this research, when the water in the crop was managed in a different way from the traditional one (permanent flooding), that is to say, provoking a condition of water stress by default, the industrial quality of the grain was favored for the years of study.

When water stress (by default) was applied to the crop in the vegetative phase, for the low rainy period during the four years of research, the stress variants significantly outperformed the control (permanent flooding) in percent of whole grains and total nitrogen in the leaf. Nevertheless, the percentage of cracked and white belly behaved in a lower percentage with water stress compared to the control, as it can be seen in Tables 1, 2, 3 and 4.

Table 1 (2013) shows that the best treatment in percent of whole grains was T₅ with 68.8%, followed by T₄ with 66.6%, the worst of the treatments being T₆ with 58, 6%. These higher percentages of whole grains in the stress variants, could be attributed to, among other causes, that these treatments also had the lower percentages of cracked grains, T₄ and T₅ with 0% cracked grains. Similarly, the lowest percentages of white-bellied grains also corresponded to treatments T₄ and T₅ with 4, 0 and 2.0%, respectively. It motivates greater percentages of whole grains, whereas the control reached 7.0 and 28.1% of cracked and white-bellied grains, respectively. Consequently, that motivates lower percentages of whole grains in the control, in relation to variants with water stress. These results differ from those reported by several researchers, when studying the industrial quality of the grain in the crop with water deficit during its development. (Alvarado and Hernaiz, 1995; Acosta et al., 2004; García et al., 2011; Osuna et al., 2013; Sarwar et al., 2013; Ndindeng et al., 2014; Polón et al., 2014).

The industrial yield, as percent of whole and cracked grains for the other dry periods (2015, 2016 and 2017) studied, remained with the same behavior as in dry season 2014. The best treatments in percent of grains (T₄ and T₅), significantly exceeded the control, which was the lowest value. That behavior is attributed to a higher percentage of whole grains in the treatments with water stress and to a null (0%) production in the percentage of cracked grains. That could also be due to the low percentages of grains with white belly, not coinciding with those reported by other authors that refer impacts in whole grains when going through periods with water stress (Cuevas y Fitzgerald, 2012; Sarwar et al., 2013; Ndindeng et al., 2014; Londero et al., 2015; Cuevas et al., 2016).

The formation of white belly in the grain for the four years studied was always lower in treatments T₄ and T₅ with water stress Tables 1, 2, 3 and 4). That allowed obtaining a greater quantity of whole grains in these two treatments per year of study. This result does not coincide with that reported in the scientific literature, which proposes a lower production of whole grains with water deficit (Thompson y Mutters, 2006; Sarwar et al., 2013). On the other hand, T₆, reached the highest values of white-bellied grains in the studied years, which contributed to the lower percentage of whole grains, as it can be seen in Tables 1, 2, 3 and 4. It seems that the condition of water stress imposed on the crop allowed for a better distribution and organization of the starch grains (more compact), reducing the parting of the grain in the mill, resulting in the obtaining of a greater amount of whole grains. Apparently, this phenomenon does not happen in the control treatment (without water stress). In this respect Acosta et al. (2004), argues that, when the starch grains are not well distributed in the grain, it creates empty spaces with air in it, which contributes to a greater parting of the grain in the mill, decreasing the percent of whole grains in a significant and important way.

As it can be seen in Tables 1, 2, 3 and 4, T₁, T₂, T_{3 and} T₄ treatments with water stress were obtained for the low rainy season, obtaining the highest percentages of industrial yield of whole grains and of better quality, compared to (T₆). It seems that when water stress condition is applied to the crop in the vegetative phase, there is an increase in the industrial yield of the grain. This coincides with that reported in bean cultivation by other researchers (Polón et al., 2003; Polón et al., 2014; Tosquy et al., 2014).

Tables 1, 2, 3 and 4 for the low rainy periods reflect the behavior of the percent of total nitrogen in the rice grain. The best treatments were T₄ and T₅ with significant differences with respect to the rest of the treatments studied, being the worst of all T₆ (control). It also corresponds to the best and worst treatment of whole grains, respectively. This behavior allowed a higher production of whole grains with better quality. It could be attributed among other causes, to the higher production of total nitrogen, when the condition of water stress is practiced to the crop, also obtaining less cracked grains and white belly in the grain, allowing smaller parting of the grain.

These results coincide with what was reported by Acosta et al. (2004), who states that the greater nitrogen presence in fertilization increases the crystal appearance of the grain, reduces cracking and splitting of the grain in the bleaching mill, increasing the quantity of whole grains in a variety of short cycle.

Conclusions

As a conclusion, it could be said that by subjecting the rice cultivation to a period of time under water stress, the percentage of whole grains is increased, the cracked grains and the presence of white belly in the grain are reduced, as well as the amount of nitrogen in the leaf is increased in J-104 middle cycle variety.